Pipetting devices are used in automatic or semiautomatic analysis instruments for clinical diagnostics, biotechnology, environmental analysis or substance analysis. Analytic methods are carried out in such analysis instruments by virtue of liquid samples and/or reagents being mixed to form a reaction mixture and the reaction being measured quantitatively, for example by photometric means. Automatically controlled pipetting devices, which usually work like a suction pipette, are used to transfer precisely metered volumes of liquid samples or reagents from storage containers to reaction containers.
Many instruments are able to pick up a multiplicity of samples and to store a multiplicity of reagents. For reasons of storage, the reagent supplies in particular are cooled in the reagent station to a temperature below room temperature. However, in order to carry out an analytical method, particularly if it is based on enzymatic reactions, it is necessary for there to be a specific temperature in the reaction mixture. It is for this reason that use has been made for a long time of warmed pipetting needles, which make it possible to adjust the temperature of the liquid to be transferred to the desired value prior to dispensing it to the reaction container. By way of example, heated pipetting devices are described in EP-A2-1134024, EP-A2-0496962 or US 2008/0044311 A1.
Modern analysis instruments are able to carry out many different tests. This means that the pipetting devices must be able to pipette different volumes of various cooled reagents and, in doing so, be able to heat these to the desired reaction temperature or to a temperature in the vicinity of the desired reaction temperature.
In known analysis instruments, use is made of pipetting devices which should constantly have the same intended temperature. In instruments for analyzing blood, plasma or serum samples, the target temperature for the reagents to be pipetted is conventionally 37° C. The temperature of the pipetting needle is measured with the aid of thermosensors. The heating device is only activated if the temperature of the pipetting needle drops below the set intended value temperature.
In practice, it is a problem to ensure that different volumes of liquid are equally heated to the same target temperature, e.g. to 37° C. Under certain circumstances, relatively large volumes are not heated sufficiently, smaller volumes are possibly overheated, or vice versa. Volumes that are so small that they only remain in the region of the suction tip of the pipetting needle are possibly not heated sufficiently either, because the region of the suction tip often is not heated at all or because there is a temperature gradient along the pipetting needle and the suction tip simply has a lower temperature than the remaining part of the pipetting needle.
In US 2008/0044311 A1, this problem is solved by virtue of the fact that the temperature in the pipetting device is matched to the volume of liquid to be transferred. The larger the volume of liquid to be transferred is, the more strongly the pipetting needle is heated. The pipetting needle is also heated more strongly in the case of particularly small volumes. A disadvantage of this method is that this requires a complicated control of the heating device and that the heating device must be embodied such that it is able to heat the pipetting needle differently from one pipetting step to the next. This particularly is a problem if a large volume of liquid which needs to be heated less strongly is initially pipetted and this is directly followed by a smaller volume of liquid which needs to be heated more strongly. In this case, there is the risk that the smaller volume is not heated enough from the still less strongly heated pipetting needle.